Machining, cutting, sawing or drilling tools are often provided with removable inserts comprising conventional materials such as cemented carbides, ceramics (e.g. Si3N4, TiC—Al2O3 composites), and high-speed steel. As depicted in FIG. 1, insert 1 is held firmly and locked into a tool holder 5 by a screw or other clamping mechanism 4. In machining operations, the insert is held in contact with the work piece and eventually wears to a point wherein it requires replacement. Inserts are, by definition, a disposable part of the machine tool system.
Superabrasive materials containing diamond (e.g. polycrystalline diamond or PCD) and/or cubic boron nitride (e.g. polycrystalline cubic boron nitride, PCBN) provide enhanced machining performance over conventional materials (tool life, surface finish, tolerance capability, etc.) and are also widely used as tool inserts. Due to the high material cost of superabrasives, fabrication techniques have been developed and optimized to reduce the usage (in mm2) of superabrasives on the insert. One such technique is the manufacture of “tipped inserts,” which is also depicted in FIG. 1. The tipped insert consists of an insert body 3 and an abrasive tip 2 of superabrasive material, with the insert body being typically fabricated out of cemented tungsten carbide. The superabrasive tip 2 is attached to a corner or edge of the insert body by a brazing process. Brazing provides sufficient binding force to withstand the cutting forces and heat and is convenient for attaching small abrasive tips.
Polycrystalline diamond (“PCD”) and cubic boron nitride (“PCBN”) are commonly manufactured bonded to cemented carbide to form a two-layer disk, with PCD or PCBN on one side and cemented carbide on the other. This is done to facilitate insert fabrication via braze attachment. The carbide side of the PCD or PCBN tips is easily brazed to carbide tip holders to make brazed inserts of the prior art. Directly brazing PCD or PCBN to carbide is challenging as the braze metal must spread over and bind quite dissimilar materials.
Although the prior art brazing process does reduce the material cost of manufacturing superabrasive inserts, the process, and in particular the brazing operation itself, is labor intensive. In this process, as illustrated in FIG. 2, an abrasive disc 11 is machined via EDM, EDG, or other processes, into a desired shape, e.g., an 80° triangle, forming an abrasive tip 12. A suitably sized pocket is ground in the insert body 13 to form the site for attaching the abrasive tip. The bottom rear edge of the abrasive tip may be chamfered to avoid stress buildup at the otherwise sharp corner where the abrasive tip will meet with the insert body 13. In the next step, a braze material, typically a metal paste, powder or foil, 14 is placed between the chamfered abrasive tip 15 and the pocketed insert body 16. A flux material may be applied to inhibit braze oxidation. The assembly is heated to a temperature above the liquidus of the braze material causing it to melt. Upon cooling, the metal freezes as a thin film binding the abrasive tip and insert body, forming the semi-finished insert, 17. The edges of the insert can then be ground to final dimensions and sharpness to produce the finished insert 18. The brazing process is labor intensive because the operator has to pay close attention to the joint interface, i.e., the abrasive tip, the braze interface layer, and the insert body, and reposition the materials, when molten, as necessary to assure good bonding. The ultimate location of the abrasive tip within the insert body and the quality of its attachment can be variable due to variable braze metal flow and the need for this manual positioning.
A difficulty in the brazing process is that tool materials of different composition or grain size frequently require different brazing conditions, i.e., temperatures, times, braze metal formulations. Additionally, brazing dissimilar materials e.g., a cubic boron nitride tip to cemented carbide insert body requires special braze alloys and conditions capable of bonding both materials simultaneously in the same process cycle. PCBN and PCD are known to be difficult to wet with brazes unless active metals, such as Ti or Fe, are incorporated into the metal formula. Such active metals are oxidation sensitive and may require use of an inert atmosphere or vacuum furnace, or very fast induction brazing, to improve the bond. They also require higher temperature that may lead to degradation of the superabrasive material.
A further disadvantage of conventionally brazed inserts is that once formed, they cannot be heated above the sublimation or liquidus temperature of the braze metal in subsequent processing steps, such as, for example, chemical vapor deposition (CVD) coating of the insert. Low melting metals used in braze alloys, e.g., Sn, Zn, are volatile and the braze bond will be impaired and/or vacuum components contaminated by thermal treatment after brazing. Additionally, damage to the abrasive tip or insert body from the thermal expansion/contraction cycle during brazing is possible, requiring brazing temperature and time to be kept to a minimum. In some cases, rebrazing tips to correct braze flaws or regrind tips is not possible. Furthermore, heat generated at the tip during cutting may damage the braze attachment, allowing the tip to displace in the holder. This will disrupt the cutting operation.
There are a number of references in the prior art for specialized tools that preclude the brazing requirements, including U.S. Pat. No. 5,829,924 titled “Cutting Tool with Insert Clamping Mechanism,” U.S. Pat. No. 4,909,677 titled “Throw Away Cutting Tool,” U.S. Pat. No. 5,154,550 titled “Throw Away Tipped Drill Bit,” and U.S. Pat. No. 4,558,974 titled “Tool System for Precision Slotting.” The prior art teachings rely on exact and complex geometrical configurations of an insert and tool holder to assure that the insert is securely gripped by the tool holder in operation. These references employ mechanical means of holding an insert in a tool holder and not holding an abrasive tip within the insert body itself.
One solution for eliminating the brazing requirements in tipped inserts was recently made commercially available and is illustrated in FIG. 3. The insert system incorporates a reusable insert body, wherein the insert body itself acts as a clamp for holding an abrasive tip. As shown, the insert body is sectioned down the central vertical plane, connecting a set of jaws on one corner to a relief hole near the opposite corner. The relief hole and central vertical cut allow reversible movement of the jaws within the horizontal plane of the insert body. The insert assembly is used with a specially designed tool holder that forces the insert back into a v-block when the clamping mechanism is tightened, squeezing the jaws on the insert body together and to firmly hold the abrasive tip in place during the machining operation. Removing the insert from the tool holder allows the insert body to spring back to its original state, allowing the jaws to open so that the tip can be removed and replaced. Abrasive tips are finish ground (i.e. chamfered, sharpened, and/or honed) before clamping into the insert body.
Because of the reusable nature of the above-describe inserts, the deformation of the insert body and resultant clamping force must be reversible. If the jaws on the insert body are pried open or forced together to the extent that the insert body material yields at the corner opposite to the jaws, it will not spring back to its original state. Replacement abrasive tips would then have to be manufactured with progressively larger (or smaller) mating features to fit the insert body, posing significant complexity in manufacturing of the abrasive tips and application of the insert system. Another disadvantage is that the design limits the insert body to holding only one abrasive tip. Thus, the insert can not be indexed, i.e. rotated in the tool holder to use another corner as the cutting edge. It must be removed and refitted with a replacement abrasive tip, increasing the down time in operation. Lastly, with the abrasive tips being ground finished separate from the reusable insert body, dimensional differences between abrasive tips are imparted to the assembled insert. This may require recalibration of the positioning of the cutting edge with respect to the workpiece every time the abrasive tip is changed, else there is a risk of imparting dimensional differences in abrasive tips to the part being machined with resulting decreased dimensional capability and increased part scrap rate.
There is a need for an improved, inexpensive, convenient, versatile insert system that eliminates the problems and costs associated with brazing without the increased complexity and limited utility of non-brazed inserts of the prior art. There is also a need for an insert that is simple and dimensionally precise, requiring minimal subsequent grinding for dimension control and sharpening. Lastly, there is a need for an insert system that enables abrasive tips and insert body materials to be selected without regard to their brazing compatibility, allows grinding of the assembled inserts, and enables post processing at high temperatures (e.g., CVD coating or hardening via thermal processing).